mpdreload: Protocol optimizations.

mpdreload: Protocol optimizations.

Try harder to send fewer commands to the MPD server.  In particular:

* We can submit commands to batch-load subranges from the target
  playlist into the play queue, then move them as a group into the
  final location.

* We don't need to send move commands for tracks that are already in
  the correct final position, which helps when reloading a playlist
  very similar to the current play queue.

* By arranging for all unwanted tracks to be shifted to the end of
  the play queue during the consolidation process, these can all be
  deleted with a single command.